The present invention relates to adhesives for use in conjunction with disposable biomedical electrodes. More particularly, the invention is directed to biomedical electrode adhesives which are of the pressure sensitive type and which are electrically conductive for establishing an electrical circuit between the skin of a patient and an electro medical device such as an electrocardiograph.
The prior art is replete with various types of biomedical electrodes which vary not only in their mechanical construction but in the use of particular body attachment means as well as electrically conductive circuits including conductive media which constitute part of the electrode assemblies themselves. The general aim of the electrodes, whether used as diagnostic devices or for electro surgery or for stimulation purposes, is to establish good electrical continuity and signal transmission between a patient's skin surface and the electrical leads connected to a specific piece of diagnostic or therapeutic equipment. Among the generally common components of prior art electrodes are structures which include a conductive support element, which may be metallic or which may have been otherwise rendered conductive, to which an electrical lead line or wire from an associated apparatus may be conveniently attached.
Many electrodes rely upon the use of a conductive electrode paste or gel which is applied or otherwise interposed as a conductive medium between the conductive suppport element of the electrode and the skin surface interface. Still other electrodes utilize a sponge or pad which is impregnated with or which contains an electrolyte solution or paste. The goal in each electrode structure is to establish a continuous and reliable contact and circuit between the skin surface of the patient and the electrical leads to the apparatus being used. Electrical artifacts are to be avoided, as are "hotspots" or localized zones of high current concentration and transmission.
Many of the prior art publications, including prior patents, are directed to particular formulations of conductive gels or pastes of the type used in conjuncton with disposable biomedical electrodes. While in a preferred embodiment of the present invention conductive gels are used to augment and enhance the corductive characteristics of the assembly and to ensure continuous contact between the conductive element of the electrode and the body skin surface, the present invention is not directed to the conductive paste or gel itself. Many gel formulations are suitable in the practice of the present invention. Such conductive gels are described in Vaughan, et al. patent application Ser. No. 555,071 entitled "Disposable Bio-Medical and Diagnostic Electrode", filed Nov. 25, 1983. The entire disclosure of that application is hereby specifically incorporated herein by reference to the extent that it is not inconsistent herewith.
The present invention is directed not to the conductive gel or to any specific "mechanical" feature of the disposable biomedical electrode but, rather, to an improved electrode conductive adhesive by means of which the electrode is secured to and retained in electrical communication with the skin of the patient.
In order to ensure a meaningful understanding of the present invention, and to enable classification of the invention in the appropriate, precise art that is relevant, certain facts pertaining to pressure-sensitive adhesives and to gels, conductive pastes, and adhesives in general are set forth herebelow. The pressure-sensitive adhesives of the present invention, as applied to biomedical electrodes can be considered to constitute modified forms of adhesives of the type used on adhesive tapes, the general purpose of which is to secure the tape backing onto the surface to which it is applied. In the present application, the function of the adhesive is not only to secure the electrode backing firmly to the patient's skin but also to provide a current path between the electrode itself and the skin surface.
In general, adhesive tapes may be divided into three principal classes depending upon the manner in which the adhesive itself is activated, whether by means of solvent, heat, or the application of finger pressure. Solvent activated tapes or "gummed tapes" are usually activated by applying water to the tape surface, the water serving as a solvent. Typical examples include tapes used for sealing cartons, envelope flaps and postage stamps. In some relatively rare industrial applications, non-aqueous solvents are used to activate the "dry" solvent-activated compositions. In all of the solvent-activated-type tapes, the adhesion increases and a bond forms as the solvent evaporates. Generally, the substrates are marred or destroyed when the tape is ultimately removed.
In adhesives of heat-activated tape, the combination of heat and pressure applied makes the adhesive sticky. Tapes such as used in garment repair or fabric mending, or for the application of "patches" on clothes are typical examples of heat-activated tapes. In this system, the adhesive bond is produced as a result of penetration and cooling, and is normally strong. The adhesion or bond is "irreversible".
Tapes in which the adhesive is activated by applying finger pressure are referred to as "pressure-sensitive tape". The adhesive itself is referred to as a pressure-sensitive adhesive (PSA). A pressure-sensitive adhesive has been defined as viscoelastic material which in solvent-free form remains permanently tacky.
These adhesives adhere instantaneously to most solid surfaces upon application of only relatively light finger pressure. PSA tapes can usually be removed cleanly from the surfaces to which they have been applied. In general, their adhesive strength is of a lower order of magnitude than that exhibited by solvent or by heat-activated tape adhesives. Typical examples of PSA tapes and adhesives are "Scotch".RTM. tape, masking tape, medical adhesive tape, Band-aids.RTM., Curad.RTM. tapes and disposable ECG electrodes.
Many different formulations of pressure-sensitive adhesives have been used for many purposes. However, such formulations ordinarily include components which find their counterpart from formulation to formulation. Such ingredients are elastomers, tackifiers, plasticizers, fillers, and antioxidants. The performance characteristics of a pressure-sensitive adhesive are dependent primarily upon three properties: tack, adhesive strength, and cohesive strength. In the manufacturing process, PSA's are ordinarily coated onto release liners or substrates at a stage in which the adhesive is either a solvent phase or an aqueous latex, or a hot melt composition. In the first two examples, the adhesive is dried by application of heat to dispel the solvent or the water present in the coated formulation. In contrast, the hot melt compositions require only that they be permitted to cool to become solid, thereby to establish the requisite tack properties. Pressure sensitive adhesive compositions are ordinarily protected by the release liner during storage and prior to their use, for example, the plastic backing on a product such as Band-aids.RTM. and Curads.RTM.. In roll form, the adhesive tape's upper surface serves as its own release liner.
It will be appreciated that neither electrode gels nor pastes come within the definition of pressure-sensitive adhesives. These products (gels and pastes) differ from each other in that the paste normally contains mineral fillers and a lower concentration of water. The ingredients present provide the paste with an opacity, a higher viscosity, and usually a greater degree of tackiness in its wet condition. While there are certain areas in which some similarity exists between electrode gels and pastes, on the one hand, and electrically conductive pressure sensitive adhesives, on the other hand, such areas are primarily in their functional capabilities. Both types of materials serve as conductive media for medical electrodes. The pressure-sensitive adhesives of the present invention serve the additional function of securing the electrode base in a close and contiguous bonded relationship with the patient's skin. Furthermore, the conductive adhesives of the present invention exhibit cohesive strength and non-drying properties allowing the adhesive coated electrode to be removed cleanly from the skin without leaving any substantial residue.
Electrically conductive pressure sensitive adhesives for use in conjunction with medical electrodes are known in the prior art. Some of these preparations utilize gums such as karaya gums which produce what may be described as homogeneous adhesives based on water soluble gelling agents. Such systems have the disadvantage of not being sufficiently compatible with electrolytes such as sodium chloride and potassium chloride. The omission of such salts from the electrolyte system impairs the utility of the final product and impairs the ability of the electrodes to be defibrillated in use. The karaya gum-type adhesives are also objectionable in that the compositions must be poured into molds and heated to produce the gelled product. Such gum systems do not tolerate significant concentrations of electrolytes.
Another class of prior art conductive pressure-sensitive adhesives used in conjunction with medical electrodes involves polymeric homogeneous systems. The polymers used in such systems are water soluble or at least strongly hydrophilic in nature. The published literature disparages applicants' type of heterogeneous system and actually "teaches away" from such a system.
Others of the prior art conductive pressure-sensitive adhesives lack specific ingredients which are believed to be necessary to enhance and maximize the efficacy of the adhesive. For example, such prior art preparations fail to include a humectant, or a humectant in a concentration sufficient to maintain and ensure conductivity within a broad range of relative ambient humidity. Such prior art preparations tend to dry-out under many ambient conditions and thereupon lose electrical conductivity.
It is, therefore, a principal aim of the present invention to provide an improved electrically conductive pressure-sensitive adhesive for use with disposable biomedical electrodes, such adhesives exhibiting markedly enhanced stability and adhesive properties and avoiding many of the shortcomings of prior art preparations.